1. Field of the Invention
The present invention relates to a distance measuring apparatus installed in an automobile to measure, for example, the distance between the road surface and the axle or vehicle body of the running automobile so as to measure the degree of inclination of the automobile or the degree of lowering of the vehicle body and thereby to detect the tire air pressure or control the optical axis of the head lamps.
2. Description of the Prior Art
Generally, ordinary distance measuring apparatus are so constructed that the apparatus comprises means for transmitting a sound wave, means for receiving the reflected sound wave from an object to be measured and means for measuring the distance to the object from the time elapsed between the transmission of the sound wave and the reaction of the reflected sound wave. To simply use this type of measuring apparatus as an automobile distance measuring apparatus which measures the degree of inclination of the automobile or the degree of lowering of the vehicle body to detect the air pressure of the tires or control the optical axis of the head lamps, a measuring accuracy of the order of 10.sup.-3 m is required. However, if the ordinary measuring apparatus is simply used to measure the distance between the axle or the vehicle body of an automobile and the road surface, the intensity of the reflected sound wave varies depending on the road surface conditions and the resulting measuring accuracy will be as low as about 10.sup.-2 m, thus making it impossible to use the apparatus in practical applications, such as, the detection of the tire air pressure, the control of the head lamp optical axis or the like.